The present invention relates to locating underground deposits, particularly to underground NMR active liquid mineral deposits, and more particularly to an instrument using earth field spin echo NMR for measuring the spatial, qualitative and quantitative parameters of an underground NMR active liquid mineral deposits, such as oil and water.
Various systems and instruments have been developed over the years for searching underground areas for minerals, oil, and water. For example, oil companies are in an active search to expand the borehole diagnostics to more than a meter beyond the borehole liner in an effort to reduce the number of production wells and improve extraction efficiency. The following patents exemplify these prior efforts: U.S. Pat. Nos. 2,999,203 issued Sep. 5, 1961; No. 3,234,454 issued Feb. 8, 1966; U.S. Pat. No. 3,597,681 issued Aug. 3, 1971; No. 4,528,508 issued Jul. 9, 1985; U.S. Pat. No. 4,646,021 issued Feb. 24, 1987; No. 4,724,385 issued Feb. 9, 1988; U.S. Pat. No. 4,792,757 issued Dec. 20, 1988; No. 4,804,918 issued Feb. 14, 1989; U.S. Pat. No. 4,933,638 issued Jun. 12, 1990; No. 4,987,368 issued Jan. 22, 1991; U.S. Pat. No. 4,988,947 issued Jan. 21, 1991; and No. 5,291,137. Also, see United Kingdom Application GB-2 198 540 filed Dec. 3, 1986. The oil reservoir topology is often very complicated, and there is a need to develop new technologies having spatial and material resolution to expand oil mapping up to 50 or even 100 meters from the borehole.
One of the prior technologies utilized to analyze oil sediments is high-field laboratory nuclear magnetic resonance (NMR), and high-field NMR probes have been utilized for downhole exploration close to the borehole. NMR was usually carried out with high magnetic fields upward of 500 Gauss to improve the signal strength from small samples.
It has been discovered that with proper hardware, as provided by the present invention, NMR can be used at low magnetic fields, but large samples are needed. Oil deposits, for example, provide a very large sample. It is impossible to create a flat magnetic field at a one-sided large distance from a coil system. Maxwell's equations do not allow such a configuration. There are a few magnet configurations to create a small close-distance flat field. Nature provides such a flat magnetic field, the earth magnetic field itself being at about 500 milli-Gauss. The related proton NMR frequency is about 2000 Hz. The field is very low by NMR standards and the natural relaxation times of protons in small pores comes close to the NMR frequency. The short relaxation time makes the Fast Fourier Transform (FFT) signal very wide. If the pore size of the rock gets extremely small (relaxation time of less than 1 ms), the signal will be lost. The low NMR frequency of protons in the earth magnetic field allows for great penetration depth of the AC driver or excitation field and the returned signal from the spinning protons. In dry rock, a useful penetration depth is 1 km, while in salt saturated wet rock, it is reduced to about 200 meters due to the limiting skin depth of conductive material. Dry salt has a reasonably low conductivity, and thus the penetration would be greater.
By the use of large diameter wire loop arrays positioned on the ground surface, and when a weak (1.5-2.5 kHz) AC field is applied, matching the NMR frequency of hydrogen in the rather flat and uniform earth magnetic field, a phased array of transmitter and receiver loops is provided, and which suppress radio and power line interference, whereby a liquid mineral deposit, oil or water, can be spatially, qualitatively, and quantitatively measured utilizing earth field spin echo NMR and magnetic resonance imaging (MRI) techniques. Also, the surface coil phased array may be combined with downhole receivers which enhance the position resolution of an MRI significantly. Mapping of areas surrounding a borehole is done by changing the excitation strength and duration of the individual driver (excitation) coils.